One way the population of the United States has grown is through the movement of individuals into the United States from other parts of the world. The same effects can be seen in wildlife populations. Individuals move into a population and increase its size. This is just one of the many factors affecting population growth. Of course, individuals may also move away from a population.

Population Growth

What does population growth mean? You can probably guess that it means the number of individuals in a population is increasing. The
population growth rate
tells you how quickly a population is increasing or decreasing. What determines the population growth rate for a particular population?

Births, Deaths, and Migration

Population growth rate depends on birth rates and death rates, as well as migration. First, we will consider the effects of birth and death rates. You can predict the growth rate by using this simple equation:
growth rate = birth rate – death rate.

If the birth rate is larger than the death rate, then the population grows. If the death rate is larger than the birth rate, what will happen to the population? The population size will decrease. If the birth and death rates are equal, then the population size will not change.

Factors that affect population growth are:

Age of organisms at first reproduction.

How often an organism reproduces.

The number of offspring of an organism.

The presence or absence of parental care.

How long an organism is able to reproduce.

The death rate of offspring.

For an ecosystem to be stable, populations in that system must be healthy, and that usually means reproducing as much as their environment allows. Do organisms reproduce yearly or every few years? Do organisms reproduce for much of their life, or just part of their life? Do organisms produce many offspring at once, or just a few, or even just one? Do many newborn organisms die, or do the majority survive? All these factors play a role in the growth of a population.

Organisms can use different strategies to increase their reproduction rate.

Altricial
organisms are helpless at birth, and their parents give them a lot of care. This care is often seen in bird species. (
Figure
below
). Altricial birds are usually born blind and without feathers. Compared to precocial organisms, altricial organisms have a longer period of development before they reach maturity.
Precocial
organisms, such as the geese shown below, can take care of themselves at birth and do not require help from their parents (
Figure
below
). In order to reproduce as much as possible, altricial and precocial organisms must use very different strategies.

(left) A hummingbird nest with young illustrates an altricial reproductive strategy, with a few small eggs, helpless young, and intensive parental care. (right) The Canada goose shows a precocial reproductive strategy. It lays a large number of large eggs, producing well-developed young.

Migration

Migration
is the movement of individual organisms into, or out of, a population. Migration affects population growth rate. There are two types of migration:

Immigration
is the movement of individuals into a population from other areas. This increases the population size and growth rate.

Emigration
is the movement of individuals out of a population. This decreases the population size and growth rate.

One type of migration that you are probably familiar with is the migration of birds. Maybe you have heard that birds fly south for the winter. In the fall, birds fly thousands of miles to the south where it is warmer. In the spring, they return to their homes. (
Figure
below
).

Monarch butterflies also migrate from Mexico to the northern U.S. in the summer and back to Mexico in the winter. These types of migrations move entire populations from one location to another.

A flock of barnacle geese fly in formation during the autumn migration.

Exponential Growth

Population growth can be described with two models, based on the size of the population and necessary resources. These two types of growth are known as exponential growth and logistic growth. If a population is given unlimited amounts of resources, such as food and water, land if needed, moisture, oxygen, and other environmental factors, it will grow exponentially.
Exponential growth
occurs as a population grows larger, dramatically increasing the growth rate. This is shown as a "J-shaped" curve below (
Figure
below
). You can see that the population grows slowly at first, but as time passes, growth occurs more and more rapidly.

Growth of populations according to exponential (or J-curve) growth model (
left
) and logistic (or S-curve) growth model (
right
). Time is plotted on the x-axis, and population size is plotted on the y-axis.

In nature, organisms do not usually have ideal environments with unlimited food. In nature, there are limits. Sometimes, there will be plenty of food. Sometimes, a fire will wipe out all of the available nutrients. Sometimes a predator will kill many individuals in a population. How do you think these limits affect the way organisms grow?

Usually, populations first grow exponentially while resources are abundant. But as populations increase and resources become less available, rates of growth slow down and slowly level off, reaching the carrying capacity. The
carrying capacity
is the upper limit to the population size that the environment can support. This type of growth is shown as an "S-shaped" curve below (
Figure
above
) and is called
logistic growth.
Why do you think occurs?

Vocabulary

altricial
: Born in a helpless state; needs a lot of parental care.

carrying capacity
: Maximum population size that can be supported in a particular area without degradation of the habitat.

emigration
: Movement of individuals out of a population to other areas.

exponential growth
: Pattern of population growth in which a population starts out growing slowly but grows faster and faster as the population size increases.

immigration
: Movement of individuals into a population from other areas.

logistic growth
: Pattern of population growth in which growth slows and population size levels off as the population approaches the carrying capacity.

migration
: Movement of individual organisms into, or out of, a population.

population growth rate
: How quickly a population is increasing or decreasing.

precocial
: Born in an independent, advanced state.

Summary

Population growth rate is affected by birth rates, death rates, immigration, and emigration.

If a population is given unlimited amounts of food, moisture, and oxygen, and other environmental factors, it will show exponential growth.

Practice

How many years did it take the human population to increase from 1 billion to 2 billion? Considering how long it took the human population to reach 1 billion, is this pattern consistent with an exponential growth model?

How fast is the human population currently increasing? What kind of growth does this indicate the human population is experiencing currently? Does this rate represent an increase or decrease from previous growth rates?

How does exponential growth affect available resources for a population? What response have we seen in other populations that experience exponential growth?

Is our current population level creating problems with available space? Why or why not? How do you think this situation should inform policy regarding problems associated with population growth?